In U.S. Pat. No. 3,559,240, there is disclosed a device for supplying materials having a low bulk density to an extruder. The material is fed into the extruder from a feed hopper by means of an auxiliary screw through a by-pass conduit. On the end of the shaft of the auxiliary screw remote from the extruder, agitator arms are mounted.
The shaft of the auxiliary screw is rotated by means of a drive and transmission unit which is separate from that of the extruder. Such feed devices may also be used for twin screw extruders.
Accordingly, such material feed devices require a separate drive and transmission unit for the shaft of the auxiliary screw which has the agitator arms mounted thereon.
Such an arrangement necessitates the use of an extremely tall building because the feed hopper is disposed vertically and the extrusion device horizontally. Accordingly, the device has an extremely large overall height.
It is highly disadvantageous if a twin screw extruder is fed through only one aperture, that is to say, outlet aperture of the feed hopper. This latter is usually disposed over the rotatable twin screws.
The highly bulky material to be fed into the extruder is compressed by the auxiliary screw and is urged through the outlet aperture in the feed hopper. It then drops into twin screws which extend in a plane at right angles to the plane of the auxiliary screw and are located below the outlet aperture. This material is thus collected by the twin screws.
The rotating twin screws can, therefore, only collect the material in one of their end regions for onward transmission.
A conventional hopper feed device, which is provided with a tamping or packing mechanism and which is disposed at right angles to the twin screw extrusion device thus leaves much to be desired. The working surface area of the rotating screws of the extruder which is available for receiving the fed material is relatively small. The working surface area of the screws can, of course, only correspond at best to the cross-sectional area of the outlet aperture of the feed hopper.
An additional disadvantage of a feed hopper which is disposed at right angles to the twin screw extrusion device and which is provided with a tamping or packing mechanism resides in the fact that the tamping action causes a downwardly acting force to be applied to the screws. Since such force must act on one end of the screws, it is found that, after a period of time, wear phenomena appears on the inner wall of the cylinders in which the screws rotate.